Treatment of silver to inhibit tarnishing



Patented July 20, 1954 TREATMENT OF SILVER T'O INHIBIT TARNISHINGCarroll F. Matthews, Constantia, and John C. Sawyer, Oneida, N. Y.,assignors to Oneida Ltd., Oneida, N. Y., a corporation of New York NoDrawing. Application October 27, 1949, Serial No. 123,982

12 Claims. 1

This invention relates to the inhibiting of tarnish on articles having asilver surface, and more particularly to the inhibiting of tarnish onsuch articles by means of certain fluoborate solutions.

in the past considerable effort has been made to prevent or to retardthe development of silver tarnish and various means have been attemptedto obtain this desired result, such as by coating the silver surfaceswith various materials. No tarnish prevention coating has yet beendevelwhich is fully satisfactory. Coating materials, such as lacquers,have the disadvantage of being relatively difficult to apply, as well asexpensive, while other methods are not sufficiently permanent or haveother disadvantages.

It an object of this invention to inhibit the formation of tarnish in arelatively simple, yet effective manner.

It is another object of this invention to inhibit the formation ofsilver tarnish for long periods of time at relatively small cost.

These and other objects will become apparent the following disclosure.

riefly, these objects may be attained by contacting the silver articleto be treated in a solution of a fiuoborate of a metal moreelectropositive than silver. An example of such a metal is tin.

in the treatment of silver articles, silver plated articles and articleswhose surface is composed of an alloy which is predominately silver, thearticle is contacted briefly with an aqueous metallic :fiuoboratesolution. Concentrations as low as 1% show marked effect whileincreasing concentrations up to about 9 per cent give increasingprotection from tarnish. Concentrations in excess of 9 per cent, whilesatisfactory, appear to be little if any more elfective than those ofabout 9 per cent.

The time of immersion can be as short as one second, although a longertime is preferred and imparts better protection. Various experimentshave shown that very satisfactory results are obtained by a 10 seconddip in a 9 per cent solution, although the time of immersion appears toof little consequence so long as the surface chemically wet.

Instead of dipping the article having a silver surface in the fiuoboratesolution, the fiuoborate may be used as an electrolyte and the articleto be treated as a cathode. In such a case a relatively small current,such as about 2 amperes per square foot, applied for a brief period issufiicient to cause the article to be very satisfactorily resistant totarnish. The current and time should be adjusted so that insufficientmaterial is plated out on the cathode to be visible.

Extensive work has shown that stannous flue-- borate is outstanding inits ability to retard tarnish and, in fact, considerably superior toother fiuoborates, although the following fluoborates have also beenfound to protect silver surfaces from tarnish. They are:

We do not know Why a brief exposure of the silver surface to an aqueoussolution of a fluoborate of a metal more electropositive than silverdecreases the activity of the silver surface. It is possible that amolecular layer of a silver compound is formed which ties up any freevalences present in surface silver atoms. Any film that forms, however,is invisible and does not affect the lustre of the silver, nor are thefilms detectible by ordinary chemical means.

A further unexplained fact is that stannous fluoborate is nearly doublyas effective as any of the other fiuoborates as measured by an arbitraryscale.

The field of commercial silverware production is largely divided betweenso-called sterling silver and plated silverware. Sterling silver is analloy consisting of 92.5 parts of silver and 7.5 parts of alloyingmetal. Other silver alloys containing varying but small amounts ofcopper, cadmium and other metals are manufactured on a lesser scale. Wehave found that we can protect silver alloy articles, of which apredominant part is silver, against tarnish by our process. ihese alloysinclude particularly silver alloy articles containing 90 per cent ormore of silver such as sterling silver. Silver plated articles i. e.articles whose surface is entirely silver, are also substantiallyprotected by our process.

The following is a series of tests given as examples to show the effectof stannous fluoborate on electroplated and on sterling silver articles.In each case the samples were first cleaned to re move grease, buffingcompounds and other surface contamination in order to permit thefiucborate solution to contact the surface.

Example I A silver plated spoon was immersed for 5 secends in a 23%solution of stannous fluoborate, removed, rinsed in clean Water anddried. To test the resistance of the treated spoon to tarnish it wasplaced in a desiccator of 9 liters capacity together with ml. ofsaturated aqueous hydrogen sulfide solution slightly acidified withhydrochloric acid for two hours; at the end of that time no tarnish wasapparent. A similar but untreated sample as a control was exposed tohydrogen sulfide at the same time and under the same conditions andbecame covered with a purple tarnish tinged with blue.

Exam le II Experiment I was repeated using sterling silver test pieces.The same procedure, solutions, etc. were followed except that thetarnishing test time was twelve minutes. Results:

Control: Golden and purple tarnish, Treated sample: Very slight tarnishon handle embossing only.

Example III Various concentrations of stannous fluoborate were preparedas indicated below and were used to show the effect of concentration onsilver plated and sterling silver spoons. The general proceduredescribed for Example I was followed in each case, using a time of dipof 5 seconds and a tarnishing exposure time of 44 minutes.

The results of this test are recorded below:

Control plated spoon-edeep blue and purple tarnish. Control sterlingspoondeep blue tarnish. Treated plated spoon:

1% SnCBFQ light brown tarnish. 4.6% Sn(BF4)2no tarnish. 9.2% Sn(BF4)2notarnish. 23.0% Sn(BF4)z.n0 tarnish. Treated sterling spoon:

1% Sn(BF4)2--blue grey tarnish. 4.6% SnCBFQ 2.-light blue tarnish. 9.2%Sn(BF4)2light blue tarnish. 23.0% Sn(BF4)z-light grey tarnish.

A comparison of the control spoons with the treated spoons shows that astartling improvement is realized by the fiuoborate dip even when theconcentration is as low as l In addition to the forced 0r acceleratedtests described above, a time test was also made.

Finishedsterling and plated teaspoons were employed for the test, andprepared in accordance with Example III, i. e. each of four sterlingsilver spoons and each of four silver plated spoons were dipped in 1%,4.6%, 9.2% and 23.0% stannous iluoborate solutions respectively and oneeach of sterling and plated spoons were selected as controls. The eighttreated and two untreated spoons were suspended by threads for two andone-half months in normal atmosphere. At the end of that time, theuntreated spoons were badly tarnished having a deep brown tarnish.

The treated plated spoons all showed some brown tarnish, but weredistinctly less tarnished than the untreated spoons. The treatedsterling spoons required close examination to detect any tarnish orchange in color.

This time test clearly shows that the fluoborate dip is of real value asa tarnish inhibiting agent.

Example IV Both electroplated and sterling silver spoons were immersedin a 4.6% stannous fiuoborate solution for five seconds. During thattime the spoons wer made the cathode of an electrolytic cell andsubjected to, an applied current density of 1.8 amperes per square foot.After treatment the spoons were rinsed, dried, and immersed in asaturated and acidified hydrogen sulfide solution for 10 minutes.Results:

The electroplated silver spoon used as a control was covered with a deepblue tarnish while the treated sample, while generally free fromtarnish, had some slight spots of brown tarnish.

The sterling control was covered with a dark blue gray tarnish ascontrasted with the treated sample which had a light gray tarnish.

While gradations are difiicult to describe in words, theelectrolytically treated samples appeared to be better thancorresponding dipped samples.

The treatment of articles having silver surfaces with fiuoborates asdescribed above finds an important use in the prevention of tarnish onarticles which are infrequently used as in jewellers displays; here sucharticles may be shown to their best advantage for long periods of time.It may also be applied as a treatment to finished silver articles priorto packaging to prevent tarnish during storage, and as a treatment toprevent tarnish to silver articles between manufacturing operations,such as occurs many times during the interval between the finishingoperations and final inspection and between final inspection andpackaging. It may also be used as a household treatment to the goodsilverware that is used only occasionally, before putting it away afterusing so that cleaning or polishing will not be required beforesubsequent use.

We claim:

1. A process for inhibiting the formation of silver tarnish whichcomprises contacting a silver tarnish-susceptible article with anaqueous solution consisting essentially or fiuoborate of a metal moreelectropositive than silver for a time sufiicient to render said articleta'nish resistant and insufficient to deposit a v detectable layer ofsaid metal on said article, said article having a surface consisting atleast in part of a metal of the group consisting of silver and alloys ofsilver wherein silver is the predominant metal, said solution having aconcentration of at least one per cent of said fiuoborate.

2. A process for inhibiting the formation of silver tarnish whichcomprises dipping a silver tarnish-susceptible article into an aqueoussolution consisting essentially of a iiuoborate or a metal moreelectropositive than silver for a time sufiicient to render said articletarnish resistant and insufficient to deposit a visibly detectable layerof said metal on said article, said article having a surface consistingat least in part of a metal of the group consisting of silver and alloysof silver wherein silver is the predominant metal, said solution havinga concentration of at least one per cent of said iiuoborate.

3. A process for inhibiting the formation of silver tarnish whichcomprises, contacting a sterling silver article with an aqueou solutionconsisting essentially of a fluoborate or" a metal more electropositivethan silver for a time sufiicient to render said articl tarnishresistant and insuilicient to deposit a visibly detectable layer of saidmetal on said article, said solution having a concentration of at leastone per cent of said fluoborate.

4. A process for inhibitin the formation of silver tarnish whichcomprises contacting a sterling silver article with an aqueous solutionconsisting essentially of stannous flucborate for a time sufficient torender said article tarnish resistant and insufiicient to deposit avisibly detectable layer of said metal on said article, said solutionhaving a concentration of at least one per cent of said fluoborate.

5. The process in accordance with claim 4 wherein the solution has aconcentration of at least nine per cent.

6. A process for inhibiting the formation of silver tarnish whichcomprises contacting an article having a, silver surface with an aqueoussolution consisting essentially of a fluoborate of a metal moreelectropositive than silver for a time sufficient to render said articletarnish resistant and insufficient to deposit a. visibly detectablelayer of said metal on said article, said solution having aconcentration of at least one per cent of said fluoborate.

7. A process for inhibiting the formation of silver tarnish whichcomprises contacting an article having a silver surface with an aqueoussolution consisting essentially of stannous fluoborate for a timesuificient to render said article tarnish resistant and insufficient todeposit a visibly detectable layer of said metal on said article, saidsolution having a concentration of at least one per cent.

8. The process in accordance with claim 7 wherein the solution has aconcentration of at least 9 per cent.

9. A process for inhibiting tarnish which comprises the steps ofpreparin an electrolytic cell, the electrolyte for said cell being anaqueous solution consisting essentially of a fluoborate, said solutionhaving a concentration of at least 1% of a fluoborate of a metal moreelectropositive than silver, and having as a cathode an article having asurface consisting at least in part of a metal of the group consistingof silver and alloys of silver wherein silver is the predominant metal,causing a current to flow in said cell but for an insuiicient time toform a visible deposit.

10. The process in accordance with claim 9 wherein the fluoborate of ametal more electropositive than silver is stannous fluoborate.

11. The process in accordance with claim 10 wherein the cathode is asterling silver article.

12. The process in accordance with claim 10 wherein the cathode is asilver plated article.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,827,2'7 Mason Oct. 13,1931 2,117,657 Ganlofi et al. May 17,1938 OTHER REFERENCES Modern Electroplating, special volume, publishedin 1942 by The Electrochemical Society; pages 311, 312 and 316.

1. A PROCESS FOR INHIBITING THE FORMATION OF SILVER TARNISH WHICHCOMPRISES CONTACTING A SILVER TARNISH-SUSCEPTIBLE ARTICLE WITH ANAQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A FLUOBORATE OF A METAL MOREELECTROPOSITIVE THAN SILVER FOR A TIME SUFFICIENT TO RENDER SAID ARTICLETARNISH RESISTANT AND INSUFFICIENT TO DEPOSIT A VISIBLY DETECTABLE LAYEROF SAID METAL ON SAID ARTICLE, SAID ARTICLE HAVING A SURFACE CONSISTINGAT LEAST IN PART OF A METAL OF THE GROUP CONSISTING OF SILVER AND ALLOYSOF SILVER WHEREIN SILVER IS THE PREDOMINANT METAL, SAID SOLUTION HAVINGA CONCENTRATION OF AT LEAST ONE PER CENT OF SAID FLUOBORATE.